I am not opposed to theoretical work. On the contrary, good theories can be extremely valuable for engineering projects. In fact, some great theoretical advances were motivated by engineering problems.
> What time limit shall we set on demonstrating usefulness of a
> theoretical idea in applications...
None. Instead, I would recommend some advice I heard in a lecture by Richard Hamming many years ago:
Paraphrase from a lecture by RH:
> When you're beginning a research project, the first thing you should
> do is to draft the first page of the report in which you present the
> final results. Then ask yourself: "If I saw that paper in a journal
> with somebody else's name at the top, would I bother to read it?"
> If the answer is no, look for another topic.